1. Field of the Invention
This invention relates to a method for controlling magnetization of a magnetic material. More particularly, it relates to an element exploiting a magnetic material, such as an information recording element for recording the information by controlling the magnetization of the magnetic material, or a variable resistance element for controlling electrical resistance by controlling magnetization of the magnetic material, and to an addressing method in an appliance employing such element.
2. Description of the Related Art
An element employing a magnetic material is attractive for two reasons as compared to a semiconductor device. First, since electrically conductive metals can be used as device elements, high carrier density and low resistance can be achieved. Therefore, an element exploiting a magnetic material is expected to be suited to design rule minuting and high integration. Second, bistable magnetization direction proper to a magnetic material can be used in a non-volatile memory. That is, if bistable magnetization direction proper to a magnetic material is utilized, a solid non-volatile memory, in which the recorded information is not lost even on interruption of the circuit power source, is expected to be realized.
Meanwhile, a solid non-volatile memory, in which the recorded information is not lost even on interruption of the circuit power source, is expected to be useful in many fields of application. Specifically, a solid non-volatile memory does not consume power during periods of non-use, and hence is expected to be in reducing the capacity and the weight of batteries in portable electronic information equipment. On the other hand, the solid non-volatile memory finds wide use, on the background of the advent of the age of satellite media business, to support the activity of a satellite while under the shade of the earth, a time when a solar battery becomes unusable.
The element exploiting the magnetic material has advantages such as i) nonvolatility; ii) no deterioration due to repeated usage; iii) possibility of high-speed writing; iv) small size and adaptability for high recording density; and v) superior resistance against radiation. These merits are discussed hereinbelow in detail.
i) non-volatility
Thanks to the bistability of the direction of magnetization proper to the magnetic material, the information written as the direction of magnetization is maintained unchanged in the absence of the driving power,
ii) no deterioration due to repeated usage
There is also proposed a memory employing a dielectric material exhibiting bistability as does a magnetic material (ferroelectric random access memory F-RAM). In this F-RAM, the memory state is rewritten by reversing the spontaneous dielectric polarization. However, since the inversion of spontaneous dielectric polarization corresponding to rewriting of the memory state is accompanied by ionic movement in a crystal lattice, repetition of rewriting over one million times leads to development of crystal defects. Thus, with F-RAM, the service life of the element, that cannot be surpassed due to fatigue of the material, poses a problem. On the other hand, since the inversion of magnetization of the magnetic material is not accompanied by ionic movement, an element exploiting the magnetic material can be used almost limitlessly for re-writing without limitation due to fatigue of the material.
iii) possibility of high-speed writing
The speed of inversion of magnetization of the magnetic material is as fast as approximately one ns, so that, by exploiting this high switching rate, high-speed writing becomes possible.
iv) small size and adaptability to high recording density
The magnetic properties of a magnetic alloy can be varied extensively subject to selection of the composition or structure. Thus, an element utilizing a magnetic material has an extremely high degree of freedom in deigning. With the element exploiting a magnetic material, it is possible to utilize electrically conductive magnetic alloy. If the electrically conductive magnetic alloy is used, the current density in the element higher than with the use of a semiconductor is assured, thus enabling further minution and higher recording density than is possible with the use of the semiconductor element.
As an element exploiting these properties, a spin transistor, as described in Journal of Society of Applied Magnetic Science of Japan, vol.19,684 (1995), has been proposed. A spin transistor has its emitter constituted by a magnetic material E, while having its collector and base constituted by a magnetic material C and a non-magnetic material B, respectively, as shown in FIG. 1. With this spin transistor, an output voltage dependent on the direction of magnetization of the magnetic materials C, E is generated by the polarization density which seeps from the magnetic materials C, E towards the non-magnetic material B. Meanwhile, the structure of the spin transistor shown in FIG. 1 is such that an output voltage depends on the direction of magnetization of the magnetic materials C and E. The direction of magnetization is changed by furnishing the current pulses for magnetization to a current line for magnetization 500 and by applying the magnetic field generated by the current pulses for magnetization P to the magnetic materials C and E.
v) superior resistance against radiations
If ionized radiations traverse an element, the memory state of which is created by charging into electrical capacitance, such as a dynamic random access memory (DRAM), electrical discharging is produced, so that the store information is lost. Conversely, the direction of magnetization of the magnetic material is not disturbed by the ionized radiations. Thus, the element exploiting a magnetic material is superior in resistance against radiation. Therefore, the element exploiting a magnetic material is particularly useful for application in need of high resistance against radiations, such as communication satellite. In actuality, a magnetic bubble memory, among the memories exploiting the magnetic material, is already finding use asa memory loaded on a communication satellite.
The device exploiting the magnetic material has many advantages, as discussed above. As a device for taking advantage of these merits, a solid magnetic memory has been proposed. The solid magnetic memory is a magnetic storage device employing an array of magnetic materials asa storage medium and, in distinction from a magnetic tape or a magnetic disc, performs the storage operation without being accompanied by movement of a storage medium.
In the conventional solid magnetic memory, a simple addressing method, exploiting the properties of the magnetic material, is used. The addressing method in the conventional solid magnetic memory is now explained.
In the solid magnetic memory, a magnetic thin film, exhibiting uniaxial magnetic anisotropy, is used. The magnitude of the magnetic field, required for inducing inversion of magnetization in the magnetic thin film, depends on the direction of application of the magnetic field. That is, inversion of magnetization can be induced with a smaller strength of the magnetic field if the magnetic field is applied in a direction inclined by approximately 45.degree. from the easy axis than if the magnetic field is applied in a direction parallel to the easy axis. In the conventional solid magnetic memory, these properties can be utilized for addressing of recording bits to enable the use of an extremely simple addressing system.
That is, in the conventional solid magnetic memory, word lines W1, W2, W3, . . . and bit lines B1, B2, B3, . . . are arrayed at right angles to one another, and storage carriers A-1, A-2, . . . , B-1, B-2, . . . , C-1, C-2, . . . are arranged at the points of intersections, as shown in FIG. 2. That is, in the conventional solid magnetic memory, storage carriers are arrayed in an x-y matrix configuration to constitute a memory chip. The easy axis of each storage carrier is aligned along the word line direction.
If the word line W2 and the bit line B1 are selected and appropriate current is fed therethrough, inversion of magnetization occurs only in a storage carrier B-1 at a point of intersection of the two lines. The word line W2 and the bit line B1, fed with the current, apply the magnetic field across plural storage carriers arrayed thereon. It is noted that the magnetic field from one of the word line W2 or the bit line B1 is insufficient to cause inversion of magnetization. It is only when a magnetic field Hw from the word line W2 and the magnetic field H, from the bit line B1 are synthesized to give the magnetic field oriented 45.degree. relative to the easy axis that inversion of magnetization is produced, that is, it is only in the storage carrier B-1 that the inversion of magnetization is produced. That is, in the conventional solid magnetic memory, the fact that inversion of magnetization is induced in the storage carrier only when the magnetic field applied across the storage carrier is oriented 45.degree. relative to the easy axis is utilized for selecting a specified storage carrier.
That is, in the conventional solid magnetic memory, a specified storage carrier can be selected to induce the inversion of magnetization using a simple arrangement of intersecting electrically conductive lines to render it possible to use an extremely simplified addressing system.
Although the elements exploiting the magnetic material has a number of merits, as discussed above, there are also presented certain demerits. The demerits produced in elements utilizing a magnetic material are explained taking an example of a solid magnetic memory. These demerits, now explained, are unexceptionally brought about due to application of the magnetic fields in the storage carriers for writing.
(i) Cross-talk
In the conventional solid magnetic memory, writing in the memory is by applying a magnetic field across the memory. However, since the magnetic field is of a force acting from a distant point, a non-negligible effect acts on an area neighboring to the selected storage carrier if the storage carrier density is high, thus producing the crosstalk. Although the designing approach of a memory cell having a magnetic field shielding structure is reported in Z. G. Wang et al, IEEE Trans Magn., Mag33, 4498 (1997), the proposed memory cell is complex in structure.
(ii) Lowered Coercivity due to Design Rule Minuting
In the conventional solid magnetic memory, the writing magnetic field is produced by the current. However, there is imposed a limit on the density of the current that can be transported by a conductor i[A/m.sup.2 ] depending on the material used. The result is that, as the design rule becomes finer and the conductor is finer in diameter, the upper limit of the current that can be used is decreased.
If the diameter of a conductor is D[m], the strength of the magnetic field H[A/m] separated at a distance L from the center of the conductor is given by the equation (1): EQU H=(.pi.iD.sup.2 /4)/4(2.pi.L) (1)
The center-to-center distance between the conductor and the storage carrier is not markedly smaller than D, so that, if L=D, the strength of the magnetic field applied to the storage carrier is given by the equation (2): EQU H=(.pi.iD.sup.2 /4)/(2.pi.L)=iD/8 (2)
If the allowable current density i is such that i=10.sup.7 [A/cm.sup.2 ]=10.sup.11 [A/m.sup.2 ] and D'[.mu.m]=D[m].times.10.sup.6, the strength of the magnetic field H applied to the storage carrier is given by the equation (3): EQU H=12500.times.D'[A/m]=156.times.D'[Oe] (3)
That is, if the magnetic material as the storage carrier is located closer to the center of the conductor by design rule minuting, in order to take account of the effect of the storage carrier approaching the source of the magnetic field, the maximum magnetic field that can be utilized is decreased substantially in proportion to the design rule value.
On the other hand, the coercivity of the storage carrier needs to be designed so that inversion of magnetization will be realized by the magnetic field applied from outside. Thus, if the magnetic field that can be applied to the storage carrier is decreased with design rule minuting, the coercivity of the storage carrier needs to be reduced correspondingly. That is, with the solid magnetic memory, the coercivity of the storage carrier needs to be reduced. However, if the coercivity of the storage carrier is reduced excessively, the operational reliability is lowered. This poses a serious problem in a memory for portable electronic equipment used in an environment subjected to a disturbing magnetic field from the ambient.
These problems inherent in the conventional solid magnetic memory arise due to application of the magnetic field across the storage carrier for writing. For overcoming these problems, it is necessary to reconsider the problems beginning from the addressing method of specifying an optional storage carrier selected as an object of writing or readout to achieve the targeted operation.
Meanwhile, the above-mentioned problem is ascribable to application of the magnetic field from outside to reverse the state of magnetization of the storage carrier and is not limited to the case of the solid magnetic memory. Similar problems arise in, for example, a spin transistor shown in FIG. 1. In the spin transistor, which realizes the function that the output is varied in dependence upon the direction of magnetization of the element constituent, an input operation, that is the operation of varying the direction of magnetization of the magnetic element taking part in output decision, is by the application of the magnetic field from the nearby current, as in the case of the above-mentioned solid magnetic memory. Therefore, the problem specified above in case of the solid magnetic memory also arises in the case of the spin transistor.
The above problem can be avoided if it is possible to control the magnetization without utilizing the magnetic field. As a technique of controlling the magnetization without using the magnetic field, there is proposed such a technique employing a ferromagnetic layer/semiconductor layer/ferromagnetic layer, layered together, as disclosed in "Mattson et al, Phys. Rev. Lett. 71 (1993) 185".
This exploits the fact that magnetic coupling between the ferromagnetic layers depends on the carrier concentration of the semiconductor layer as an intermediate layer. In the ferromagnetic layer/semiconductor layer/ferromagnetic layer, layered together, magnetic coupling between the ferromagnetic layers can be changed from parallel to anti-parallel, as an example, by controlling the carrier concentration of the semiconductor layer as an intermediate layer. Thus, if the coercivity of one of the magnetic layers (fixed layer) is increased, it is possible to rotate the magnetization of the opposite side magnetic layer (movable layer) with respect to the fixed layer. This technique, which enables rotation of magnetization by an electrical input, is viewed as promising as a technique of realizing the samll-sized solid-state device.
Meanwhile, in the ferromagnetic layer/semiconductor layer/ferromagnetic layer, layered together, there is produced indirect magnetic interaction between the ferromagnetic layers via the semiconductor layer. In order to control the magnetic coupling between the ferromagnetic layers by controlling the carrier concentration of the semiconductor layer as the intermediate layer, it is necessary to reduce the film thickness of the semiconductor layer as the intermediate layer.
The reason is that the magnitude of the interaction between the ferromagnetic layers via the semiconductor layer is attenuated exponentially with respect to the thickness of the semiconductor layer. For realizing a realistic magnitude of the interaction, the coercivity of 1000 Oe is accorded by, for example, exchange biasing method, to a piece of a Ni--Fe alloy having a thickness of 2 nm and saturation magnetization of 12500 Gauss. For according an energy equivalent to the energy required for inverting the magnetization of the Ni--Fe alloy by the indirect interaction via the semiconductor layer, it can be estimated, by simple calculations, that the exchange coupling constant need to be not less than 0.02 erg/cm.sup.2. From the thesis by J. J. de Vries, entitled "Physical Review Letters" 78 (1997) p.302.sup.3, it is seen that the separation between the ferromagnetic layers needs to be approximately 2.5 nm. That is, in order to provide a practically useful element, the thickness of the semiconductor layer needs to be 2.5 nm or less.
It is however not realistic in the current fine working technique to prepare an element using a thin film not larger than 2.5 nm in thickness. Moreover, if such element could be actually prepared, the semiconductor laser of this order of thickness is thought to be acting substantially as an insulation barrier due to the formation of a depletion layer brought about by the formation of a Schottky barrier on an interface between the semiconductor and the ferromagnetic layer. Therefore, it is difficult to implant carriers.
Consequently, an element comprised of the ferromagnetic layer/semiconductor layer/ferromagnetic layer, layered together, cannot be prepared without significant difficulties, although it is theoretically possible to control the magnetization without employing the magnetic field.